Hepatitis A virus (HAV) is a human pathogen that continues to pose a worldwide health problem, especially in developing countries. The virus is an atypical member of the picornavirus family, and appears to represent a unique genus. Although it shares the same general structure and genome organization with other picornavirsuses, it has only limited nucleotide or amino acid sequence homology to members of any of the other picornavirus groups. In addition, numerous aspects of HAV growth and replication in cultured cells differ markedly from those of other picornaviruses. Whereas other picornaviruses undergo rapid, lytic growth cycles, HAV replicates slowly and asynchronously and establishes persistent infections. We have previously initiated studies to identify and characterize the molecular biology of HAV in order to understand its growth phenotype. In addition, we have produced HAV proteins in recombinant systems to evaluate the feasibility of a subunit type vaccine for this infection. In this application, we propose to determine the structures of the HAV caps id proteins and their precursors, and to analyze the roles of specific HAV gene products and sequences as viral growth determinants. Capsid protein processing and modification will be defined by both biochemical and genetic methods. The 5' noncoding region will be analyzed for its role in translational efficiency and host range restriction, and the viral proteins, RNA sequences and biochemical activities that regulate viral RNA synthesis will be evaluated.